The present invention relates to a method for controlling the level of liquids, and in particular to a method that can be employed for controlling the level of liquids contained in vats with free surfaces and/or tanks under pressure, for instance those present in the semiconductor manufacturing systems. The present invention also relates to an apparatus for carrying out said method.
Apparatuses for controlling the level of liquids are known to comprise a control device provided with a pressure sensor connected to a measure tube immersed in the liquid to be controlled. The value measured by the pressure sensor, divided by the value of the density or specific weight of the liquid, determinates its level.
However, these known apparatuses provide approximate results since the values measured by the pressure sensor are influenced by the local pressure, for example by the atmospheric pressure or by the pressure in a tarn under pressure, which can vary during the time.
It is therefore an object of the present invention to provide a method and an apparatus for controlling the liquid level, which are free from this disadvantage. Said object is achieved with a method and an apparatus, the main features of which are specified in claims 1 and 6, respectively, while other features are specified in the remaining claims.
Thanks to the measurement of the pressure in an auxiliary duct and to the calculation of the relevant difference with the pressure in the measure tube, the method and the apparatus according to the present invention allow to obtain measurements of the liquid level more precise than the known methods and apparatuses.
According to an advantageous aspect of the invention, the apparatus can correct the value of the density or specific weight of the liquids according to their temperature, so as to further improve the measurement precision.
According to another advantageous aspect of the invention, the apparatus can be provided with a single pressure sensor and with an electric valve for alternatively measuring the pressure in the measure tube and in the auxiliary duct. With this arrangement, the manufacturing costs are decreased and errors due to measure differences with a further pressure sensor are avoided.
According to a farther advantageous aspect of the invention, the measure tube of the apparatus can be connected to a source of gas under pressure, so as to measure the counter-pressure corresponding to the pressure exerted on the gas by the liquid. With this arrangement, it is avoided that noxious vapors released by the liquid can reach the control device through the measure tube.
According to another advantageous aspect of the invention, the apparatus is provided with electronic memories and calculation means which serve for determining the liquid level and for controlling the calculation parameters, as well as other useful parameters for the correct working of the apparatus. Furthermore, with this arrangement, the apparatus according to the present invention can control not only the level of the liquid contained in a vessel, but also the working of a rinsing vat, with further decrease of the costs when the apparatus is employed in a semiconductor manufacturing system.
For decreasing the manufacturing and maintenance costs of the apparatus, said electronic calculation means can advantageously consist in a programmable logic controller of the known kind, which can also be connected to an external computer for the remote control of the apparatus according to the present invention.